A pulmonary alveolus (plural: alveoli , from the Latin alveoli , "small cavity") is a hollow cavity found in the lung parenchyma, and the unit basic ventilation. The lung alveoli is the tip of the respiratory tree, branched from an alveolar sac or alveolar duct, which like the alveoli is a gas exchange site with blood as well. Alveoli specifically for the lungs of mammals. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas exchange surface. Blood-rich carbon is pumped from the whole body to the capillaries surrounding the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed.
Video Pulmonary alveolus
Structure
The alveoli is located in the respiratory zone of the lung, at the end of the alveolar duct and the alveolar sac, representing the smallest unit in the respiratory tract. They provide a total surface area of ​​about 75 m 2 .
A typical pair of human lungs contains about 700 million alveoli, yielding 70m 2 surface area. Each alveolus is wrapped in a fine capillary net covering about 70% of its extent. The adult alveolus has an average diameter of 200 micrometers, with an increase in diameter during inhalation.
Microanatomy
Alveoli consists of layers of epithelial and extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between the alveoli called Pores of Kohn. Alveoli contains several collagen fibers and elastic fibers. The elastic fibers allow the alveoli to stretch as they are filled with air during inhalation. They then bounce back while inhaling to dissipate the carbon dioxide-rich air.
There are three main types of cells in the alveolar wall - two types of alveolar cells (also called pneumocytes) and large phagocytes known as alveolar macrophages.
Type I cell
Type I cells are thin and flat and form an alveoli structure. Type I alveolar cells are squamous (giving more surface area to each cell) and cover about 90-95% of the alveolar surface. Type I cells are involved in the gas exchange process between the alveoli and the blood. These cells are very thin (sometimes only 25'nm) - electron microscopy is needed to prove that all alveoli are covered with epithelial lining. These cells must be very thin to be easy on the surface as it allows easy gas exchange between the alveoli and the blood.
Organelles of type I alveolar cells such as endoplasmic reticulum, Golgi and mitochondrial apparatus gather around the nucleus. The nucleus occupies a large area of ​​free cytoplasm. This reduces cell thickness, thereby reducing the thickness of air-blood barrier. The thin cytoplasm contains pinocytotic vesicles that may play a role in the removal of small particulate contamination from the outer surface. In addition to desmosomes, all type I alveolar cells have a junctional junction that prevents leakage of tissue fluid into the alveolar air space.
Type I pneumocytes can not replicate and are susceptible to toxic pests. If damage occurs, type II cells may proliferate and differentiate into type I cells to be compensated.
Type II cells
Type II cells secrete pulmonary surfactants to lower surface water tension and allow the membrane to separate, thereby increasing its ability to exchange gas. Surfactants are continuously released by exocytosis. It forms a hypophase containing a water-containing protein and a phospholipid film on top of which consists mainly of dipalmitoyl phosphatidylcholine. Type II alveolar cells cover a small portion of the alveolar surface area. Type II cells are also capable of dividing cellular, giving rise to more type I and II alveolar cells when lung tissue is damaged. These cells are granular and coarsely cuboid-shaped. Type II alveolar cells are usually found in the air-blood barrier. Although they only reach & lt; 5% of the alveolar surface, they are relatively large (60% of alveolar epithelial cells).
Macrophages
Alveolar macrophages are also called dust cells destroying foreign objects and microbes such as bacteria.
Maps Pulmonary alveolus
Function
Thin type I cells, flat cells lining the alveolar walls. Each alveolus is surrounded by many capillaries, and is a gas exchange site, which occurs by diffusion. The relatively low solubility (and therefore the diffusion rate) of oxygen requires large internal surface area (about 80 square meters [96 square meters]) and very thin alveoli walls. The weave between the capillaries and help to support them are the extracellular matrix, the elastic fiber fabric and the netlike collagen. Collagen fibers become more rigid, giving the wall firmness, while the elastic fibers allow for expansion and contraction of the wall during breathing.
The II type cells in the alveolar wall contain secretory secretory organelles known as lamellar bodies that fuse with the cell membrane and secrete lung surfactants. This surfactant is a fatty substance film (the majority of dipalmitoylphosphatidylcholine), a group of phospholipids that reduce the alveolar surface tension. Phospholipids are stored in a flat body. Without this layer, the alveoli will collapse and enormous strength will be needed to extend it again. Type II cells begin to develop in about 26 weeks of gestation, secreting a small amount of surfactant. However, a sufficient amount of surfactant is not secreted until about 35 weeks of pregnancy - this is the main reason for the increased rate of respiratory infant respiratory syndrome, which drastically reduces the age above 35 weeks of gestation.
Type II pneumocytes may replicate in the alveoli and will replicate to replace damaged type I cells.
MUC1, the human gene associated with type II pneumocytes, has been identified as a marker of lung cancer.
Other types of cells, known as alveolar macrophages, reside in the internal surfaces of alveolar air cavities, alveolar ducts, and bronchioles. They are mobile scavengers that function to engulf foreign particles in the lungs, such as dust, bacteria, carbon particles, and blood cells from injury.
Respiratory alveoli reinflation is made easier by the lung surfactant, which is a mixture of phospholipids and proteins that reduce surface tension in the thin liquid layer in all alveoli. The liquid layer is produced by the body to facilitate the transfer of gas between blood and alveolar air. Surfactants are produced by type II cells that are the most numerous cells in the alveoli, but do not cover as much as the surface area of ​​the squamous alveolar cells (squamous epithelium).
Source of the article : Wikipedia
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